We know, there’s no such thing as a stupid question. But there are definitely some questions too embarrassing to ask your local shop or riding buddies. AASQ is our bi-weekly series where we get to the bottom of your questions – serious or otherwise.

Question #1: Hi, I’m using a Schwinn stingray. Size is 20 × 4 1/4. Is it same as 4.25? Been looking hard, but I’m yet to find one after my tube burst and I’m looking for a replacement. Thanks! – Vader

Bikerumor: Over the history of the bicycle, as a rule of thumb, tires labeled with fractions are not interchangeable with their equivalent size in decimal form (i.e. 26 x 1 1/2″ does not equal a 26 x 1.5″ tire). Tubes however, are a bit of a different story. Since they’re essentially a round balloon, they’ll stretch to fit within a reasonable size range. In this case though, it doesn’t really matter since the 20 x 4 1/4″ size in both tires and tubes seems to be readily available. They’re sold under the Sunlite brand which should be easy to get from any bike shop with a J&B account.

Question #2: What’s the purpose for and how tight should the little plastic crankarm bolt on Shimano cranks for? I usually get it a bit more than finger tight but I’m wondering if that’s too much or not enough? – Ryan

Bikerumor: Officially, that little plastic piece is called the Hollowtech II Left Hand Fixing Cap. Its job is to pull the two halves of the crank together, so that the left crank arm sits in the right place on the spindle and there aren’t any gaps between the arms and the bottom bracket seals. Shimano states that the cap should be torqued to 0.7-1.5 Nm or 6-13 in-lbs. If you’re just using your fingers and not a TL-FC16, TL-FC18, or similar tool, you’re probably not getting it tight enough which could cause premature wear of the BB. However, if you’re saying that you’re using your fingers to turn the TL-FC16 (the little top hat shaped tool) rather than a torque wrench, then you’re probably fine. I typically use the TL-FC18 without a torque wrench and tighten it until I’m sure the crank arms have contacted the BB seals, then back if off a hair since it offers more leverage than the smaller TL-FC16.

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15 COMMENTS

  1. Officially, its called a tensioner bolt or crank arm fixing bolt. Thats how its listed on Shimano exploded view diagrams.

  2. Also, that little flappy plastic key thingie that sits in the split between the two sides needs to fit into its spot. If you go too far or not far enough, the key won’t hit the hole in the spindle, and you’ll be improperly preloaded.

  3. “You’re going to want to tighten this between finger-tight and slightly more finger-tight.”
    This system has always been a joke. If it were more important, they’d give you a hex cap like SRAM. 1Nm torque readings are so prone to error with standard bike shop torque wrenches. Just be happy if you’re using Hollowtech; premature wear will happen 10x slower than BB30 on a good day anyways.

    • It’s pretty important. Non-drive arms come off fairly often, even when installed with an accurate, low torque, torque wrench.

      • I’ve never seen a Shimano crank arm come off that had been properly torqued. I’ve seen many come off when they had been installed by people that insisted they could “feel” that the bolt was between 12 and 14Nm, or didn’t go back to the first bolt after tightening the second…

    • What “standard” bike shop torque wrench covers 1nM, and slightly below, torque value? I’ll use a CDI torque screwdriver for stuff like that, but it’s not a standard tool by any means.

      I’ve always understood the stated value by Shimano to be a way to quantify what we usually call “just right”. It’s seems mainly for factory settings where quick assembly, without “feel”, is desired.

    • this basically means “make it as tight as a headset preload bolt” which is to say- adjust for proper bearing preload. it’s about as much of a joke as cup and cone hubs or threadless headsets.
      Sram gxp and nearly all bb30 bottom brackets are a captured bearing system. once it’s fully snug it’s in the optimal position for bearing preload. pressfit gxp and sram bb30 generally have a wave washer to make up for frame tolerances. some systems (i.e. rotor, some quarq ) often have a little plastic knob to turn, which ostensibly does the same thing as the shimano preload cap- that is, set bearing preload.

  4. Brown’s Law Of Tire Sizing:

    If two tires are marked with sizes that are mathematically equal,
    but one is expressed as a decimal and the other as a fraction,
    these two tires will not be interchangeable.

What do you think?